Look at their financial statement. It only has 28 million dollars as the member capital. Is that correct?
IB is part of the IB Group (formally called The Timber Hill Group). The Timber Hill Group is very well capitalized. For a history of the Group see: http://www.interactivebrokers.com/html/companyInfo/history.html Note the firms long history, auto-liquidation process, abilty to not allow trades unless sufficient margin is available, etc. should give some comfort. I'm not on the accounting side but it seems to make sense not to have a great deal of idle capital sitting around in a brokerage. All funds are fully segregated and the firm and/or affiliates are members in good standing on over 30 exchanges around the world.